1. Field of the Invention
The present invention relates to a method for preparing a chlorinated vinyl aromatic polymer, and more particularly to the preparation of a chlorinated vinyl aromatic polymer (hereinafter referred to simply as "chlorinated polymer") having solvent resistance, fusion resistance, heat resistance and fire resistance and having substantially the same molecular weight distribution as the molecular weight distribution of the starting material vinyl aromatic polymer (hereinafter referred to simply as "polymer") by dissolving the polymer in an organic solvent and subjecting it to an electrolytic reaction in an aqueous solution containing chlorine ions.
2. Description of the Prior Art
The vinyl aromatic polymer is widely used as a molding material since it is readily processable and has good transparency, colorability and electric properties. However, it is inferior in the solvent resistance, heat resistance and fire resistance. It has been an important objective in this technical field to improve such properties.
On the other hand, it is known to improve the properties of the polymer by chlorinating it. For instance, it is known to chlorinate the polystyrene in a solvent such as carbon tetrachloride in the presence of a catalyst such as benzoyl peroxide with use of a chlorinating agent such as chlorine, N-chlorosuccinimide or sulfuryl chloride (For instance, Japanese Unexamined Patent Publication No. 16995/1973 and U.S. Pat. No. 3,812,061).
However, such a method has a drawback that depolymerization takes place during the chlorination treatment whereby the molecular weight of the polymer will be substantially lowered, and the mechanical properties of the polymer will thereby be degraded (e.g. Journal of Applied Polymer Science, Vol. 12, p. 2065).
Under the circumstances, it has long been wanted to develop a technique capable of imparting to the polymer fire resistance, fusion resistance and heat resistance by chlorination of the polymer starting material without impairing the desirable properties of the polymer starting material.